1. Field of the Invention
This invention relates to zinc phosphate coatings for metal surfaces and to processes for phosphatizing a metal surface with acidic aqueous phosphate solutions. The invention is applicable to a variety of substrates including cold rolled steel (xe2x80x9cCRSxe2x80x9d), zinc alloys, and aluminum.
2. Statement of Related Art
Present day phosphate coating solutions are generally dilute aqueous solutions of phosphoric acid and other chemicals which, when applied to the surface of an active metal, react with the metal surface to form on the surface of the metal an integral layer of a substantially insoluble amorphous or coating. Generally, the crystalline coatings are preferred.
Typically the solutions include phosphate ions, zinc and other metal ions, especially manganese, nickel, and other divalent metal cations, to provide specific characteristics desired in the final coating. Other ions typically present may be nitrate, nitrite, chlorate, fluobotate or fluosilicate. A typical phosphating process comprises the following sequence of process steps: (1) cleaning and conditioning; (2)phosphating itself; and (3) post treating. Rinses are generally employed between each of the noted steps to prevent or at least reduce carry over of materials to the next step.
Despite advances in both the composition of the phosphate coating solution and the phosphating process, there is a continued demand for still further improvements in the compositions and processes, in order to provide more control over the process, assure adequate coating weights, reduce formation of scale or white spots, and reduce adverse environmental impact and safety hazards.
In this description, except in the claims and operating examples or where explicitly otherwise indicated, all numbers describing amounts of ingredients or reaction or usage conditions are to be understand as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred.
Also, unless expressly stated to the contrary:
percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight;
the term xe2x80x9cpolymerxe2x80x9d includes oligomer;
the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred;
description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed;
specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole (any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible, otherwise such counterions may be freely selected; and
except for avoiding counterions that act adversely to the stated objects of the invention): and the term xe2x80x9cmolexe2x80x9d and its variations may be applied to elemental, ionic, and any other chemical species defined by number and type of atoms present, as well as to compounds with well defined molecules.
It has now been discovered that certain zinc phosphate compositions containing both nickel and manganese along with hydroxylamine sulfate (xe2x80x9cHASxe2x80x9d) as an accelerator provide polycrystalline coatings and retain the advantages of the use of manganese and nickel and the accelerator properties of the HAS, without changing the platelet or needle like crystalline morphology, as described in U.S. Patent 4,865,653. The HAS accelerated zinc phosphating compositions of the present invention produce a desirable uniform gray manganese and nickel modified zinc phosphate coating on a variety of substrates including ferrous alloys, zinc alloys and aluminum and its alloys at desirable temperatures in the range of 38 to 66xc2x0 C., preferably 46 to 54xc2x0 C., and can be applied by both spray and/or immersion and by any other method that establishes contact between the compositions and the substrates. The hydroxylamine sulfate accelerator can be incorporated into the makeup and replenishing mixtures, when needed, without the need of traditional supplemental accelerators, such as nitrite, which are undesirable because of their chemical instability and consequent unsuitability to single package concentrates from which a complete working composition can be prepared by dilution with water only. Compositions according to the invention ate also highly tolerant of sulfate ions, which may be introduced into the compositions during use.
In addition to providing overall desirable advantages, without many of the disadvantages previously encountered in the art, the present invention provides for improved process uniformity at the low temperature and reduces adverse environmental impact and safety hazards associated with nitrite. The polycrystalline coating contains Zn, Mn and Ni in the coating, and Fe in coatings on ferrous surfaces.
The present invention also includes as one embodiment a make-up or concentrate composition, which may then be diluted with water to form an aqueous, acidic coating solution for a spray or immersion coating process.
In general, a composition according to the invention, used for actual contact with a metal substrate to form a phosphate conversion coating, preferably will contain, more preferably will consist essentially of, or still more preferably will consist of water and:
The stoichiometric equivalent as fluoride ions of any content of complex fluoride ions such as fluosilicate, fluotitanate, fluobotate, and the like is to be understood as included within the content of fluoride ions for the purpose of the preferred concentration ranges noted above.
xe2x80x9cThe coating solution may be formed by diluting a concentrate. The concentrate is accordingly formulated to provide a coating solution preferably containing, more preferably consisting essentially of, or still more preferably consisting of water and:
(A) from 0.5 to 2 g/l, more preferably from 0.8 to 1.2 g/l of zinc ions;
(B) from 5 to 25 g/l, more preferably from 10 to 15 g/l, of phosphate ions;
(C) from 0.2 to 1.5 g/l, more preferably from 0.5 to 1 g/l, of manganese ions;
(D) from 0.2 to 1.5 g/l, more preferably from 0.5 to 1 g/l, of nickel ions;
(E) from 1 to 2.5 g/l, more preferably from 1.5 to 1.75 g/l, of hydroxylamine accelerator; and, optionally,
(F) up to 1.5 g/l of total fluoride ion with, more preferably, a free fluoride content of 400-600 parts per million; and, optionally,
(G) up to 2 g/l of nitrate ions; and, optionally,
(H) up to 14 g/l, more preferably no more than 9.0 g/l, still more preferably no more than 6.0 g/l, of sulfate ions.
In the phosphating solutions, it is preferable that the weight ratio of zinc ion to phosphate ion be in the range from 1.0:10 to 1.0:25 and the weight ratio of zinc to the sum of manganese and nickel be in the range from 1.0:0.5 to 1.0:2.5, with the ratio of manganese to nickel being most preferably 1.0:1.0, with a ratio in the range from 1.0:0.5 to 1.0:1.5 being satisfactory.xe2x80x9d
The xe2x80x9cfree fluoride contentxe2x80x9d noted above is defined and can conveniently be measured by means of a fluoride sensitive electrode as described in U.S. Pat. No. 3,431,182 and commercially available from Orion Instruments. xe2x80x9cFree fluoride contentxe2x80x9d as this term is used herein was measured relative to a 120E Activity Standard Solution commercially available from the Parker+Amchem (xe2x80x9c+Axe2x80x9d) Division of Henkel Corporation, Madison Heights, Mich. by a procedure described in detail in P+A Technical Process Bulletin No. 968. The Orion Fluoride Ion Electrode and the reference electrode provided with the Orion instrument are both immersed in the noted Standard Solution and the millivolt meter reading is adjusted to 0 with a Standard Knob on the instrument, after waiting if necessary for any drift in readings. The electrodes are then rinsed with deionized or distilled water, dried, and immersed in the sample to be measured, which should be brought to the same temperature as the noted Standard Solution had when it was used to set the meter reading to 0. The reading of the electrodes immersed in the sample is taken directly from the millivolt (hereinafter often abbreviated xe2x80x9cvxe2x80x9d or xe2x80x9cmVxe2x80x9d) meter on the instrument and convened to parts per million by comparison with the millivolt readings obtained with solutions of known free fluoride content
In the phosphating compositions of the present invention, it is desirable for the solution to have a total acidity of 15 to 25, more preferably 17-22, most preferably 19-20, along with a free acidity of 0.3-1.0, more desirably 0.4-0.8, and most preferably 0.5-0.7. Acidity herein is expressed in points, in which by xe2x80x9cpointsxe2x80x9d as used herein is meant the milliliters (xe2x80x9cmlxe2x80x9d) of 0.1 N NaOH required to titrate a 10 ml aliquot sample, to a pH of 8.2 with phenolphthalein indicator for total acid and to a pH of 3.8 with bromophenol blue indicator for free acid.
Suitable and preferred sources of the ingredients of the phosphating solutions of the invention include the following: for zinc ions: zinc oxide, zinc carbonate, zinc nitrate, etc.; for phosphate ions: phosphoric acid, zinc phosphate, zinc monohydrogen phosphate, zinc dihydrogen phosphate, manganese phosphate, manganese monohydrogen phosphate, manganese dihydrogen phosphate, etc.; for manganese ions: manganese oxide, manganese carbonate, manganese nitrate, the above manganese phosphate compounds, etc.; for nickel ions: nickel oxide, nickel nitrate, nickel carbonate, etc.; for fluoride ions: hydrofluoric acid, fluoboric acid, fluosilicic acid, fluotitanic add, ammonium bifluoride, and their metal salts (e.g., zinc salt, nickel salt, etc.; for nitrate ions: nitric acid, nickel nitrate etc.).
Hydroxylamine is the essential accelerator in compositions according to the present invention and can be added to the concentrate before dilution to the coating solution. The hydroxylatine caa be added in any suitable form, including any conventional source. The term xe2x80x9chydroxylamine acceleratorxe2x80x9d as used herein means any compound, such as a hydroxylamine salt or complex, that provides hydroxylamine in the compositions, usually by dissociation as the relatively small amount of free hydroxylamine in equilibrium with the hydroxylamine agent is consumed by the chemical reactions that produce the desired phosphate coating on the treated metal substrates. Suitable examples of hydroxylamine accelerators include hydroxylamine phosphate, nitrate, or sulfate, or mixtures thereof. More preferably, the hydroxylamine source is HAS, a stable form of hydroxylamine.
As stated above, the metal surfaces treated in accordance with the present invention include iron-based surfaces, zinc-based surfaces, aluminum-based surfaces, and their respective alloy-based surfaces. These metal surfaces can be treated either separately or in combination. Some advantages of the present invention are most prominently exhibited when the treatment is carried out on metal surfaces which include both an iron-based surface and a zinc-based surface, as, for example, on a car body.
It is conventional to perform other steps before and after the improved phosphating step of the present invention. Thus, it is advantageous to take steps to see that the part, workpiece or other article to be coated is substantially free of grease, dirt, or other extraneous matter. This is preferably done by employing suitable cleaning procedures and materials known to those skilled in the art. These would include, for example, mild or strong alkali cleaners, acidic cleaners, and the like. Such cleaners are generally followed and/or preceded by a water rinse.
It is highly preferred to employ a conditioning step following or as part of the cleaning step. These conditioning solutions which are known to the art typically employ titanium phosphate compounds, and preferably including condensed phosphate(s).
After the coating is formed by application of the compositions of the invention, the coated article is preferably rinsed with water and dried. The drying may be accomplished by simple ambient air drying but a forced air drying at elevated temperatures often may advantageously be employed. In the coating step the temperature is preferably maintained at 46xc2x0 to 54xc2x0 C., although temperatures up to 66xc2x0 C. are sometimes employed. At lower temperatures, longer time periods are typically required to achieve a uniform coating. Treatment times may vary from 30-180 seconds dependent on the temperature and technique of application.
Practical and preferred embodiments of the invention can be further illustrated by means of the following examples, which are not intended as limiting the invention.